Gyrator Test Mule: 4P1L Preamp

It’s always great to come back and revisit a great design. The 4P1L preamp performs flawlessly so I tweaked the gyrator board to see how it worked with the BF862 FET. The result is great, it sounds as good as it measures:

The 4P1L is biased to 150V/25mA which is the maximum current that the BF862 can do (IDSS max). You can see that the frequency response is flat up to 1.5MHz. The LF response of my test mule is affected by the AC coupling of the measuring gear. However it should be around 5-10Hz.

The distortion of low-level signals is really good:

Predominantly H2, it’s very nice to see THD<0.015% for a 4Vrms output. The load is 100KΩ which is the typical input impedance of an amplifier (with exception of solid state gear)

This low distortion manifests across the entire audio band (ignore the THD below 20Hz which is a byproduct of my testing gear):

The nice thing to see also, it’s how well the 4P1L can drive larger voltage swings:

We can see H4 popping up, however odd harmonics are lower (H5 in fact is higher than H3). THD at 10Vrms is still below 0.03%!

27 Preamp

An IHT preamp, oh yes!

I always loved the 27 valve. It was one of the first line stages I built many years ago before adventuring in the DHT world. I still have a large collection of them and I was very fond of the mesh anode ones. Please check Thomas’ blog in which he wrote a very nice note about it.

With the hybrid mu-follower (a.ka. gyrator) configuration, we can build a minimalistic and great preamp stage. The 27 has a mu of 9, so in some scenarios this may be a bit too much gain, but for many cases, it’s just what we need to drive the valve amps. Someone recently asked me for help on this, so here it goes my version:

The circuit is dead simple. The 27 is biased with a battery via a grid leak resistor (R1). C1 blocks DC from input and contributes to LF response by forming a pole with R1. 150nF is good enough but if you don’t have any, use 220nF. The operating point is 6mA looking at my old notebook. The supply doesn’t need any funky regulation, and 180-200V should do. The top FET should be either DN2540 or any other depletion of your choice. The lower JFET should be either a 2SK170GR or 2SK170BL (preferably). You can use a J310 here as well (or SMD BF862).

The sound is beautiful and THD is very low driven by H2 only, as you would expect from this triode.

If you don’t want battery bias, you can add a 1K5 resistor in the cathode with its decoupling cap and remove the battery and C1. R1 should be changed to 47k then.

Hope you enjoy this!

Ale

CX371a / 71a DHT Preamp

More than 4 years ago I ran a lovely 71a preamp which sounded amazing. I used it for some time and enjoy its sound up until I continued with my exploration around DHT preamps. Recently I was asked about how to implement this lovely valve again.

The CX371a / 71a valve is a great candidate for a line stage with its low mu and anode resistance. In my experience you have to run it above 20mA and over 100V to get the best out of this valve:

The implementation of this preamp is dead simple and a few components are needed on top of the gyrator PCB:

I haven’t starved the filaments as I found this valve not to be microphonic. If you have an 01a preamp you can modify it slightly. The interesting thing is that you can run it with just 180V. Even 150V should work and you need 25mA on each channel. A J310 or BF862 lower JFET device will work fine and you will need a heatsink for the top device (e.g. DN2540). Filament resistor is anything close to 50Ω. I used some 51Ω Russian NOS wire wound resistors, but any combination will be fine.

Enjoy

Ale

Russian PSE in Steroids (01a into 4P1L) – Part IV

More power

Our previous west meets east circuit can be improve further. In fact, a compromise made with the filament bias design is that coupling between driver (FET follower) and the output stage wasn’t DC. We want DC coupling to get best performance, to ensure we can drive well the output stage and provide sufficient grid current even when not operating in A2. This can be done with filament bias, however, since we are already introducing a negative supply, I’d prefer removing the filament bias and go for proper grid bias to get best performance of output stage in terms of maximum power and linearity.

The below circuit can be easily implemented with just few modifications from previous version:

What has changed here? Not much, the coupling cap C2 is now between the gyrator and the FET follower. The gate bias resistor R6 provides high impedance to the gyrator load to ensure maximum performance of the 01a driver (minimum distortion given size of load). Not as good as previous version, but good enough. The R6 is connected to a potentiometer which sets the bias voltage. The bias voltage is derived from V2, the -50V negative supply. You can see that this circuit will put more stress into the M1 FET as now there is an additional 25V of drop across it so power burned on this device increases.

The output of the follower is directly coupled (DC) to the output stage. The filament bias resistors are removed and we use the Coleman regulators directly on the filaments of the 4P1L.

This amplifier responds better to the grid current of the output stage once the output power goes over 3.5W. At 4.5W the distortion is just above 3% (3.2%) with a 3Vpp input signal. A tad more and you can get to the 5W and a bit more into A2 operation.

Russian PSE in Steroids (01a into 4P1L) – Part III

From Russia with Love

East-West Divide, Copyright by Justmeans

The interesting combination to explore from our previous designs is to mix some western valves like 01a into the Russian parade.

The result would be quite interesting, as the sound of the 01a has proven to be amazing. Therefore 01a driving 4P1L is possible as the 4P1L doesn’t need a lot of drive. Instead of using 4P1L as a driver, we can opt for the 01a which has a similar gain. What is interesting is that the voltage swing required by 4P1L wouldn’t force the 01a outside the zone in which is highly linear, hence, with some modifications, it can work as a great driver here.

The circuit

Instead of starving the filaments of the 01a, given the voltage swing requirements for a driver, we ought to drive it at full tilt. In the circuit above, the 01a hasn’t got the stones to drive the 4P1L pair, therefore we have added a cathode follower as explained here. The M1 follower will then drive easily the output stage.

Russian PSE in Steroids (6E5P into 4P1L) – Part II

Well, it was obvious I couldn’t leave last post as it was. There is an option to change the driver for a different valve. You can use a C3m (low gain in triode mode which is ideal here), a C3g, E180F/E280F. 6S45P or my loved 6E5P (or 6E6P) as the driver. Not longer a 100% DHT, but a nice option for sure. The 6E5P is extremely linear, good driver, with a nice gain (μ=30) in triode – perhaps more than enough for a 4P1L stage and would help in avoiding additional filament supplies.

The 6E5P has curves not dissimilar to the 4P1L as no further distortion cancelation can be seen. Here is the updated schematic if you’re interested in playing with:

Again, the gyrator PCB can be easily used to simplify the build of this amp. The 6E5P is not driven hard, but at a nice current of 20mA which makes the driver operate in a linear region (and with good sound) with just a pair of red LEDs. The nearly 30dB of gain will make this amp to be very sensitive. The 5W can be easily achieved with 1Vpp, so you will need to have an attenuator, no preamp needed clearly. The 6E5P will drive an 300B nicely here which needs the voltage gain, not like the 4P1L.

As you can see, there are plenty of option to try on this 4P1L PSE amplifier.

Russian PSE in steroids (4P1L into 4P1L)

Introduction: DHT madness

I’m not going to dwell on DHT sound. I’d rather say that if you’re looking for a stellar DHT candidate, the 4P1L beats them all. It’s dirty cheap, reliable and sounds amazing. You can go any route you like, it’s your own decision of course. However if you’re looking for a 100% DHT amp to build, here is an interesting example for your consideration.

I’ve tried 4P1L in many topologies. The advantage of its low filament requirements is that you can implement it in filament bias and simplify the circuit significantly.

The gyrator driver using the PCB I designed recently, can be used to avoid iron and have an excellent first stage and make this Russian Amp in steroids: 4P1L driving 4P1L.

One minor caveat around 4P1L in excess. I have found (as well as many others) that if you use too many 4P1L stages (e.g. 4P1L line stage driving a 4P1L-4P1L amp) then it will sound a bit harsh in the treble. i suspect this may be explained due to the H3 component level when triode-strapped. I’d rather limit the number of 4P1L stages to two. You’ve been warned.

The other great thing about the 4P1L is that is quite consistent between samples and easy to match pairs. Also in PSE mode you can drive it to full tilt with only 20Vrms and achieve up to 5W in class A1 with a pair of valves.

Continue reading “Russian PSE in steroids (4P1L into 4P1L)”